"Long before it's in the papers"
January 27, 2015


Parasitic ant may have evolved from its own host, at home

Aug. 22, 2014
Courtesy of the University of Rochester
and World Science staff

A group of par­a­sit­ic ants seem to have evolved in­to a sep­a­rate spe­cies from the very ants that they free­load on, with­out ev­er leav­ing the col­o­ny, a study has found.

If con­firmed, the phe­nom­e­non would be not only bi­zarre, but al­so a chal­lenge to tra­di­tion­al ver­sions of ev­o­lu­tion­ary the­o­ry, which hold that spe­cies rarely evolve apart while liv­ing to­geth­er.

“Most new spe­cies come about in ge­o­graph­ic isola­t­ion,” said bi­ol­o­gist Chris­tian Ra­bel­ing of the Uni­vers­ity of Roch­es­ter in New York, one of the re­search­ers. But “we now have ev­i­dence that specia­t­ion can take place with­in a sin­gle col­o­ny.” 

Specia­t­ion is the pro­cess in which spe­cies branch apart to form sep­a­rate spe­cies, like a family tree. It oc­curs when mem­bers of a spe­cies are no long­er able to re­pro­duce with mem­bers of the an­ces­tral spe­cies. But this is gen­er­ally thought to be un­likely to hap­pen un­less ge­o­graph­ic bar­ri­ers, such as moun­tains, keep the two groups sep­a­rate.

The par­a­sit­ic ant is found only in a sin­gle patch of eu­ca­lyp­tus trees on the São Paulo State Uni­vers­ity cam­pus in Bra­zil. 

Ra­bel­ing and colleagues, who pub­lished their find­ings Aug. 21 in the jour­nal Cur­rent Bi­ol­o­gy, said their work re­veals an ex­am­ple of “sym­patric specia­t­ion,” which oc­curs when a new spe­cies de­vel­ops in the same ar­ea as its par­ent spe­cies. “While sym­pat­ric specia­t­ion is more dif­fi­cult to prove,” said Ra­bel­ing, “we be­lieve we are in the pro­cess of ac­tu­ally doc­u­ment­ing a par­tic­u­lar kind of ev­o­lu­tion-in-progress.”

Since Dar­win’s time, “ev­o­lu­tion­ary bi­ol­o­gists have long de­bat­ed wheth­er two spe­cies can evolve from a com­mon an­ces­tor with­out be­ing ge­o­graph­ic­ally iso­lat­ed from each oth­er,” said Ted Schultz, cu­ra­tor of ants at the Smith­so­ni­an’s Na­t­ional Mu­se­um of Nat­u­ral His­to­ry and co-author of the stu­dy. “With this stu­dy, we of­fer a com­pel­ling case for sym­pat­ric ev­o­lu­tion.”

The par­a­sit­ic ant spe­cies, called My­co­cepu­rus cas­tra­tor, lives with and off of its host ants, My­co­cepu­rus goeldii. The host ants cul­ti­vate fun­gus to feed them­selves, while the par­a­site ants eat the fun­gus with­out do­ing any of the gar­den­ing. The re­search­ers stud­ied ge­net­ic rela­t­ion­ships of all fun­gus-growing ants in South Amer­i­ca to de­ter­mine wheth­er the par­a­site did evolve from its host. They found that the par­a­sites were, in­deed, ge­net­ic­ally very close to M. goeldii, but not to the oth­er ant spe­cies.

They al­so con­clud­ed that the par­a­sit­ic ants were no long­er re­pro­duc­tively com­pat­ible with the host ants—mak­ing them a un­ique spe­cies—and had stopped re­pro­duc­ing with their host 37,000 years ago—a short time in ev­o­lu­tion­ary terms.

The re­search­ers said they found a big clue by com­par­ing the ants’ genes, both in the cell’s nu­cle­us and in mi­to­chon­dria, or energy-prod­ucing com­part­ments in cells. Genes are made of un­its called nu­cleotides. Ra­bel­ing found that the se­quenc­ing of those nu­cleotides in the mi­to­chon­dria is be­gin­ning to look dif­fer­ent from what is found in the host ants, but that the genes in the nu­cle­us still have traces of the rela­t­ion­ship be­tween host and par­a­site, lead­ing him to con­clude that M. cas­tra­tor has be­gun to evolve away from its host.

But Ra­bel­ing said that the findings are not yet confirmed because not all the genes have been sequenced; that work is in progress.

The par­a­sit­ic ants need to be dis­creet be­cause free­loading is a big no-no in ant so­ci­e­ty, the sci­en­tists ex­plained. Of­fend­ers have been known to be killed by ant mobs. As a re­sult, the par­a­sit­ic queen of the new spe­cies has evolved in­to a smaller size, mak­ing her hard to tell apart from a reg­u­lar ant. And where­as host queens mate in an aer­i­al cer­e­mo­ny while fly­ing in rainy weath­er, the par­a­sit­ic queens mate dis­creetly in the nest. This means the two groups can’t sex­u­ally in­ter­act.

* * *

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A group of parasitic ants seem to have evolved into a separate species from the very ants that they freeload on, without ever leaving the colony, a study has found. If confirmed, the phenomenon would be not only bizarre, but also a challenge to traditional versions of evolutionary theory, which hold that species rarely evolve apart while living together. “Most new species come about in geographic isolation,” said biologist Christian Rabeling of the University of Rochester in New York, one of the researchers. But “we now have evidence that speciation can take place within a single colony.” Speciation is the process in which species branch apart to form separate species, like a family tree. It occurs when members of a species are no longer able to reproduce with members of the ancestral species. But this is generally thought to be unlikely to happen unless geographic barriers, such as mountains, keep the two groups separate. The parasitic ant is found only in a single patch of eucalyptus trees on the São Paulo State University campus in Brazil. Rabeling and the research team, who published their findings Aug. 21 in the journal Current Biology, said their work reveals an example of “sympatric speciation,” which occurs when a new species develops in the same area as its parent species. “While sympatric speciation is more difficult to prove,” said Rabeling, “we believe we are in the process of actually documenting a particular kind of evolution-in-progress.” Since Darwin’s time, “evolutionary biologists have long debated whether two species can evolve from a common ancestor without being geographically isolated from each other,” said Ted Schultz, curator of ants at the Smithsonian’s National Museum of Natural History and co-author of the study. “With this study, we offer a compelling case for sympatric evolution.” The parasitic ant species, called Mycocepurus castrator, lives with and off of its host ants, Mycocepurus goeldii. The host ants cultivate fungus to feed themselves, while the parasite ants eat the fungus without doing any of the gardening. The researchers studied genetic relationships of all fungus-growing ants in South America to determine whether the parasite did evolve from its host. They found that the parasites were, indeed, genetically very close to M. goeldii, but not to the other ant species. They also concluded that the parasitic ants were no longer reproductively compatible with the host ants—making them a unique species—and had stopped reproducing with their host a mere 37,000 years ago—a very short period in evolutionary terms. The researchers said a big clue was found by comparing the ants’ genes, both in the cell’s nucleus as well as in the mitochondria—the energy-producing structures in the cells. Genes are made of units called nucleotides. Rabeling found that the sequencing of those nucleotides in the mitochondria is beginning to look different from what is found in the host ants, but that the genes in the nucleus still have traces of the relationship between host and parasite, leading him to conclude that M. castrator has begun to evolve away from its host. Rabeling explained that just comparing some nuclear and mitochondrial genes may not be enough to demonstrate that the parasitic ants are a completely new species. “We are now sequencing the entire mitochondrial and nuclear genomes of these parasitic ants and their host in an effort to confirm speciation and the underlying genetic mechanism.” The parasitic ants need to be discreet because freeloading is a big no-no in ant society, the scientists explained. Offenders have been known to be killed by ant mobs. As a result, the parasitic queen of the new species has evolved into a smaller size, making her hard to tell apart from a regular ant. And whereas host queens mate in an aerial ceremony while flying in rainy weather, the parasitic queens mate discreetly in the nest instead. As a result, the two groups can’t sexually interact.